A spark plug is described in German Patent No. 2106 893 A1, for instance. The spark plug has a housing in which there is an insulator. A longitudinal bore has been put into the insulator in which a center electrode is situated. Three ground electrodes are fixed to the housing, one of the side electrodes being designed as a top electrode and the other two side electrodes as laterally placed electrodes. By the application of an ignition voltage, a spark gap forms between the center electrode and one of the side electrodes. The spark gap between the top electrode and the center electrode runs along a longitudinal axis of the longitudinal bore of the insulator (spark air gap). Between the laterally placed electrodes and the center electrode, a surface gap forms, which runs over the end face of the insulator facing the combustion chamber. The center electrode is situated fitting precisely into the longitudinal bore of the insulator, or has only a slight distance from the insulator. Such spark plugs, in which, because of the electrode geometry, both a spark air gap and a surface gap (or rather a surface air gap) are able to form, are used particularly in applications in which strong carbon fouling of the insulator may occur. This is the case, for example, during use in stratified-charge engines. Because of the spark discharge via the surface gap, the soot on the surface of the insulator is at least partially combusted.
What is disadvantageous about this is that soot deposits on the insulator at the start of the internal combustion engine, since the insulator is heated up only slowly during the starting process.
Because of a carbon-fouled surface of the insulator, a so-called sliding discharge, that is, a discharge between housing and insulator, is favored specifically in the starting phase, since, during the starting phase, particularly high ignition voltages are present because of a lower intake-manifold vacuum, later ignition and lower intake temperature. Such a sliding discharge may lead to problems during ignition of the air/fuel mixture in the combustion chamber and may also cause ignition misfiring.